Microwave detector



Jan. 6, 1953 w. D. HERSHBERGER MICROWAVE DETECTOR Filed May 28, 1945 BY@A w M f m j M m m wr., m :m /Z wm v M m n W 0? p m www 1 m n Wn .o f amw M MMM .w mfr/6 Patented Jan. 6, 1953 MICROWAVE DETECTOR William D.Hershberger, Princeton, N. J., assignor to Radio Corporation of America,a corporation of Delaware Application May 28, 1945, Serial No. 596,243

(Cl. Z50- 27) l Claims.

This invention relates generally to microwave transmission systems andmore particularly to an improved method of and means for detectingmicrowave energy.

The instant invention is an improvement upon and includes modificationsof the device disclosed and claimed in applicants copending applicationSerial No. 540,429 led June 15, 1944, which issued October 4, 1949 asPatent No. 2,483,768, and assigned to the same assignee as the instantapplication. In said copending application, a microwave-absorptive-gasfilled cavity resonator, which is electrically resonant to a microwavecarrier frequency and acoustically resonant to a modulation frequencysuperimposed upon said microwaves, is employed to convert the modulationcomponent of the microwaves to acoustic waves which are radiated from adiaphragm comprising one wall of the cavity resonator.

The various modications of the instant invention utilize the conversionof modulated microwaves into acoustic waves in the microwave absorptivegas enclosed Within the cavity resonator to apply pressure variations toa piezo crystal or to a pressure responsive diaphragm or other pressureresponsive device for generating electric potentials characteristic ofthe microwave modulation.

A first embodiment of the invention utilizes a cavity resonatorenclosing a microwave absorptive gas, wherein one of the resonator wallsis an electrode of a quartz piezo crystal. Pressure variations appliedto the piezo crystal electrode provide mechanical distortion of thecrystal, resulting in electric potentials between the crystal electrodescharacteristic of the microwave modulation. The potentials derived fromthe piezo crystal electrodes are amplied in any conventional manner andapplied to any desired utilization device such, for example, as anoscilloscope, an indicator, a meter, a counter or other electricalutilization device. For example the instant invention provides anextremely efficient and rugged indicator for a radar receiver system.Such an indicator has the advantage that it is substantially protectedagainst overload signals which often occur in radar systems.

A modication of the instant invention includes means for tuning thecavity resonator by means of tuning stubs located in the transmissionline connecting the modulated microwave source to the cavity resonator,and a conductive diaphragm terminating one end of the ,resonator forapplying pressure variationsendwise to a Rochelle salt piezo crystal forderiving electric potentials from the electrodes thereof characteristicof the microwave modulation. The Rochelle salt type of piezo crystalprovides much greater sensitivity to weak microwave modulated signalsthan the quartzpiezo crystal since the piezo activity of Rochelle saltis much greater than that of quartz.y

A second embodiment of the invention utilizes a gas-filled tuned cavityresonator which is electrically resonant to a microwave carrierfrequency and acoustically resonant to a modulation frequency componentof said microwaves wherein the pressure variations produced in themicro- Wave absorptive gas enclosed Within the resonator actuate amicrophone, or other pressure responsive device, for generatingelectriccurrents which may be amplified and applied to a utilization device ofthe type described heretofore. Any type of microphone may be employedwhich is responsive to pressure variations or to displacement of aportion of the resonator wall.

The e'ciency of the conversion process from microwave energy to acousticenergy may be as high as 34 percent, depending upon the type of couplingto the acoustic device and the efficiency of the microwave irradiationof the gas enclosed within the resonator. This may be shown from thefollowing formulas wherein an increment of heat dQ generated -bymicrowave absorption in the gas is wherein dU is the incremental changeof internal energy in the gas, p is the gas pressure, and v is thevolume of the gas.

Since in the instantk device the volume is held co-nstant, (10:0, and

wherein dU represents the increase in the thermal energy in the gas atconstant volume. This may be expressed as (3) dU=Co dT wherein Cv is thespecific heat of the gas at constant volume and dT is the increment ofabsolute temperature.

For a perfect gas (4) p2J=RT wherein R is the universal gas constant or(5) pdv-{-vdp=R 'dT but since wherein Cp is the specific heat atconstant pressure, and y is the ratio of specic heats wherein 'Udprepresents the potential energy of the acoustic standing wave in theinterior of the which for ammonia (NH3) is 34 percent.

The high frequency response limits of the devices disclosed herein maybe determined either by the mechanical constants of the crystal andresonant acoustic cavity system, which may be partially controlled byproper design, or by the time limitations for the exchange of energybetween the internal degree of freedom of the gas responsible formicrowave absorption and the external degrees of freedom of the gas. Thelatter limitation is fundamental in devices of this type and may limitthe high frequency response to. from 105 to l07 cycles per second,depending on the lifetime of the excited state of the molecules of thegas.

Among the objects of the invention are to provf vide an improved methodof and means for detecting microwave energy. Another object in the.invention is to provide an improved microwave detector for modulatedmicrowave energy. An additional object of the invention is to provide'animproved microwave detector for modu lated microwave energy wherein themodulated microwaves are converted to acoustic pressure variations whichactuate a pressure responsive detector. A further object of theinvention is to provide an improved microwave detector comprising acavity resonator which is electrically revscnant to a microwavefrequency and which is acoustically resonant to a modulation componentof said microwave frequency.

Additional objects of the invention include improved means for detectingmodulated microwaves comprising a cavity resonator enclosing a microwaveabsorptive gas for generating pressure variations therein characteristicof the modulation of saidmicrowaves. Another object is to provide animproved microwave detector employing a piezo crystal for generatingelectric potentials responsive to4 pressure variations induced in amicrowave absorptive gas subjected to modulated microwave irradiation. An additional object is to provide an improved microwave detector formodulated microwave energy comprising a cavity resonator enclosing amicrowave absorptive gas subjected to modulated microwave irradiation,and a microphone or other pressure or translational responsive deviceactuated by pressure variations induced in the microwave absorptive gas.A further object of the invention is to provide improved meansforcoupling modulated 4 microwave energy into a cavity resonator which iselectrically resonant to the microwave frequency and acousticallyresonant to the modulated microwave frequency for deriving pressurevariations characteristic of the microwave modulation component.

The invention will be described in greater detail by reference to theaccompanying drawing of which Figure l is a cross-sectional,partiallyschematic view of a first embodiment of the invention; Figure 2is a cross-sectional, partiallyschematic view of a modification of saidfirst embodiment of the invention; and Figure 3 is a cross-sectional,partially-schematic view of a second embodiment of the invention.Similar reference characters are applied to similar elements throughoutthe drawing.

Referring to Figure l a modulated microwave generator I is coupledthrough a waveguide 3 into one end of a cavity resonator 5 having amicrowave permeable window 'I opening into the waveguide 3. The cavityresonator 5 is propor tioned to be electrically resonant to the carrierfrequency of the microwaves and acoustically resonant to the microwavemodulation frequency. Conductive projections 9 extending from op-aposite walls of the cavity resonator intermediate the ends of theresonator provide for electrical resonance in the portion of theresonator adjacent the waveguide window while substantially reducingmicrowave coupling to the remaining portion of the cavity resonator.However, the opening between the projecting elements 9 provideseiiicient acoustic coupling between the two end portions of theresonator.

The end of the resonator 5 remote from the microwave permeable windowv'I is terminated by one electrode Il of a quartz-piezo crystal havingits electric axis parallel with the longitudinal axis of the cavityresonator 5. The piezo crystal I3 also includes a second electrode I5 onthe opposite crystal face from the rst electrode II. The crystalelectrodes may be plated or evaporated upon the opposite faces of thecrystal. The crystal electrode H terminating the cavity resonator 5 isgrounded thereto and the other crystal electrode I5 is connected througha transmission line I'I to the input of an amplifier I9. The amplifierI9 amplies the modulation component of the modulated microwaves forapplying operating potentials to a utilization device 2| of any of thetypes described heretofore.

Input potentials applied to the amplifier ISare generated between thecrystal electrodes I'I= and I5 dueto. mechanical pressure variationsapplied to the crystal in response.` to acoustic pressure variationsestablished in: the. cavity Vresonator 5 due to. microwave irradiationof: a microwave absorptive gas enclosedtherein; A sample of themicrowave absorptigve gas. for example ammonia,V may be conned'withinthe rescnator atl suitable Dres/sure., or the gas maybe continuouslycirculated thrcugh the resonator- 5 by means of4 an inletport 23 and anoutlet` port 25.4

The structure of the crystalA support may be varied in anyconventionalmanner lzrovid'ingv the crystal electrode II is maintained4 in goedcontact withthe adjacent end of the c'avityresonator 5'. For example aninsulating' ring or plate 2l may surround the endsV of thei crystal,and" the crystal electrode I 5; may lbe subjectedto pressurel by'meansoll acoiled springA 29enclosed3within the crystal housing 3l.

A modification off the device described byref--y erence to Figure 2includes microwave tuning plugs 33 and 35 mounted on the transmissionwaveguide 3 adjacent to the microwave permeable window 1, for tuning theresonator through the window v'I for matching the resonator to themicrowave transmission system. Also the quartzpiezo crystal I3 may bereplaced by a Rochelle salt crystal I3' which is end-driven by aflexible conductive diaphragm 31 closing the end of the cavity resonator5 remote from the microwave permeable window 1. The crystal electrodesll and I 5 are plated upon or supported against the wide opposite facesof the crystal normal to the driving diaphragm 31. 'I'he crystal issupported in position and electrical contact is made to the crystalelectrodes by means of spring contacts 39 and 4|. The entire crystalcontact and electrode assembly is maintained in contact with thediaphragm 37 at suitable pressure by means of the coil spring 2Senclosed within the crystal casing 3|.

The output of the Rochelle salt piezo crystal is coupled through anampliiier to a utilization device in the same manner as describedheretofore with respect to Figure l. 'I'he Rochelle salt type of piezocrystal provides much higher output potentials in response to mechanicaldeformation of the crystal and hence provides a much more sensitivemicrowave detector than the quartz crystal type. However, the Rochellesalt type of piezo crystal is subject to wide operating variations dueto absorption of moisture from the atmosphere. Such types of crystalsalso are easily damaged by mechanical shock and hence require much morecareful handling than piezo crystals of the quartz type.

'Ihe tuning plugs 33, 35 may be of the type described in my copendingapplication Serial No. 537,960 filed May 29, 1944, providing Vernieradjustment of the position of the tuning piston with minimum noisesignal introduction due to erratic sliding contact resistance betweenthe piston and the tuning plug wall.

The device illustrated in Figure 3 is similar in all respects to thedevices illustrated in Figures 1 and 2 with the exceptions that theresonator 5 is constructed of suicient cross-sectional area toaccommodate the translational element of any conventional type ofmicrophone 45 which is supported by the resonator side walls to form oneend wall of the resonator remote from the microwave permeable window 1.The microphone 45 may be of the pressure-responsive ortranslationalresponsive type actuated by the pressure variationsproduced in the microwave absorptive gas coniined within the resonator 5due to modulated microwave irradiation. Currents or voltages derivedfrom the microphone 45 are applied through the amplifier I9 to theutilization device 2l to actuate said device in response to themicrowave modulation component of the irradiating microwaves.

Thus the invention disclosed comprises a cavity resonator confining amicrowave absorptive gas for detecting the modulation component ofmodulated microwave signals. The modulated microwaves produce pressurevariations in the microwave absorptive gas which actuate a piezocrystal, a microphone or other pressureor translationalresponsive deviceto generate electric potentials characteristic of the microwavemodulation.

I claim as my invention:

l. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means for introducing microwave energy to bedetected into said resonator to vary the pressure of said gas therein,and an electrical translating 6 device subjected to said gas pressurewithin said. resonator for directlyconverting said pressure variationsinto electrical energy characteristic of said microwave energy. f

2. A microwave detector comprising a cavity' resonator enclosing amicrowave absorptive gas, means for introducing microwave energy to bedetected into said resonator to vary the pressure of said gas therein,and a piezoelectric device subjected to said gas pressure within saidresonator for directly converting said pressure variations intoelectrical energy characteristic of said microwave energy.

3. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means for introducing microwave energy to bedetected into said resonator to vary the pressure of said gas therein,and an electro-acoustical device subjected to said gas pressure withinsaid resonator for directly converting said pressure variations intoelectrical energy characteristic of said microwave energy. i

4. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means for introducing microwave energy to bedetected into said resonator to vary the pressure of said gas therein,and a piezoelectric device having one electrode disposed coincidentallywith the inner wall of said resonator and subjected to said gas pressurefor directly converting said pressure variations into electrical energycharacteristic of said microwave energy.

5. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means for introducing microwave energy to bedetected into said resonator to vary the pressure of said gas therein, apiezoelectric device having one electrode disposed coincidentally withan inner wall of said resonator and subjected to said:` gas pressure andhaving another electrode dis-- posed outside of said resonator fordirectly converting said pressure variations into electrical energycharacteristic of said microwave energy, and an amplifier responsive tosaid piezoelectric device.

6. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means including a microwave permeable windowin said resonator for introducing microwave energy to be detected intosaid resonator to vary the pressure of said gas therein, a piezoelectricdevice having one electrode disposed coincidentally with an inner wallof said resonator and subjected to said gas pressure and having anotherelectrode disposed outside of said resonator for directly convertingsaid pressure variations into electrical energy characteristic of saidmicrowave energy, and an amplifier responsive to said piezoelectricdevice.

7. A microwave detector comprising a cavity resonator enclosing amicrowave absorptive gas, means for introducing modulated microwaveenergy to be detected into said resonator to vary the pressure of saidgas therein at the modulation frequency, and an electrical translatingdevice subjected to said gas pressure within said resonator for directlyconverting said pressure variations into electrical energycharacteristic of said microwave modulation.

8. A microwave detector comprising a cavity resonator electricallyresonant to a microwave carrier frequency and acoustically resonant to acarrier modulation frequency, said resonator enclosing a microwaveabsorptive gas, means for introducing modulated microwave energy to bedetected into said resonator to vary the pressure 9- A: microwave",detector: comprising a cavity,

resonator electrically resonant toV4 af microwave carrien vfrequencyand: acousticallyl resonant to f a carriermodulation; frequency,sa'idresonator en: closingI a1, microwave absorptivev gas, means'r in?cluding;V a ,microwave permeablev window'fin` said resonator forintroducing modulated. microwave energy-,to begdetected into;saidresonatorto vary thepressure-of` said, gasatherein at Vsaid modulationfrequency, meansr ifor 'l tuningA said-'1 window.v

andfan electrical translatingdevice subjected to'sadgas:pressurewithinfsaidresonatorfor-directa lyconvertingsaidfpressure` variationsV into-ele ctricalenergy characteristic ofsaidzmicrowaveimodulation.A

10. A microwave detector-comprising; a cavity resonator acousticallyresonantwto awmicrowave carrier modulation frequency; said Yresonatorenclosing a microwave absorptive gas, means irr-v cluding a microwavepermeable-window enclosing one end of said resonatorHfor introducingmodulated microwave energy to be detected into said resonator to varythe pressure of said gas therein at said modulationfrequency, aconductive Y-diaphragm-enclosing-another end of said resonator, and apiezoelectric device coupled to said diaphragm for directly convertingsaid pressure variations h,into electrical energy; characteristic or"said microwave modulatiom,

11. Ar microwave Adetectorcomprising a cavity resonator electricallyYresonant .to a microwave carrier*frequencyY and 'acoustically resonanttoa carrier-modulation frequency-said resonator enclosing amicrowave,absorptive gas, means including aglmicrowave permeable windowenclosinggoneyendof saidY resonator fory introducing modulatedmicrowaveenergy to be'detected into said; resonator to vary the pressure of saidl.gas therein at said modulation frequency, a con.- ductive diaphragmenclosing another end ofvsaid resonatoryanda piezoelectric-deviceend-coupled to said diaphragm for directlyconverting saidpressure-variationsyinto electrical energyV characteristic of saidmicroweme modulation.f

guide. enclosing a. microvsfavev absorptive' gasg.

rneans` for introducing: microwave energy toi be detected intosaid'waveguide to varyftheepressure of said gas therein, .and "anelectrical ,translatingI device subj ecte'd" to said.; gas .f pressure'within! said resonator for directly converting. said'. pressurevariations intoi electrical energy,y characteristic ofl saidrmicrowaveenergy.'

14; Af microwave c detectorfcomprisingia wave guide having a gas-'tight'section i therein '-en'closlv ing a1. microwave absorptive igas, ,meansfor` in# tro ducing microwave v energyf to 'be' 2 detectedv into saidwaveguide to vary the pressure of said gas within said gas-tightsection,.anda`nf. electrical translating device subjected to' said' gas'pressure within said resonator for 'directly convertingsaid pressurevariations into electrical energy'` char'- acteristic of saidmicrowaveenergy;

15. A microwave detector comprising. a wave conduit enclosing amicrowave absorptivey gas, means for introducing,microwave'energy to bedetected into said conduit to vary;,the pressure of said gas therein,and an electrical `.translating .device subjected to said'gaspressurewithin said resonator for directly converting, said pressure variationsinto electrical energy characteristic of said microwave energy.

WILLIAM D. HERSHBERGER.'

REFERENCES CITED 7 The following references areof recordvin the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,123,242 Hellman .July12, 19382,372,193 Fiskr Mar\.*27, 1945 OTHER REFER-lilNGESfv Ammonia AbsorptionMeasurements Y with Guided Waves and the Shape `of a-"Special Line, byI-I. S. Howe.

